The possible instabilities can
be explored using Eq. (REF ). According to the known
mathematical theorems about the stability of the fixed points of
differential equations, linear stability guarantees non-linear (at
least perturbative) stability for sufficiently small perturbations.
In the present case, this point was confirmed in
[1]}, [2]} for the Bianchi-I metric, that is a
reduced form of the perturbation for \({{\bf k}}=0\) .
